This invention relates to a manual or automatic microwave tuner to be used mainly in harmonic load pull testing of power transistors by being able to synthesize the amplitude and phase of selected impedances at each harmonic frequency independently.
Modern design of high power microwave amplifiers and oscillators, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is inadequate for the transistors operating in their highly non-linear region, close to power saturation, to be described using non-linear analytical or numerical models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (FIGS. 1 and 2).
When transistors operate in their non-linear range at high power and close to saturation, signal distortion inside the transistors creates significant harmonic frequency signal components that reduce the efficiency and the signal transmission purity of the communication system. In order to improve and optimize the performance of such transistors under these operating conditions, the tuners used to test these DUT must also provide for independent control of the impedance at the harmonic frequencies, or “harmonic impedances”. If such tuners are used, then the load pull system described above becomes a “harmonic load pull system” (FIGS. 1 and 2).
There are essentially two methods that allow generation and manipulation of microwave impedances presented to the DUT:
A. Using electro-mechanical [1] or passive electronic tuners [2], leading to “passive load pull” (FIG. 1); and;
B. Active tuners, leading to “active load pull” [3], (FIG. 2).
Electro-mechanical tuners [1] have a number of advantages compared to active tuners, such as long-term stability, higher handling of microwave power, easier operation and lower cost. Such tuners use adjustable mechanical obstacles (probes) inserted into the transmission media of the tuners in order to reflect part of the power coming out of the DUT and to create a “real” impedance presented to the DUT.
Passive electronic tuners have been used in the past, but they provide limited tuning range and handling power and do not offer any significant benefit within the scope of this invention.
Existing passive electro-mechanical tuners, as used in set-ups shown in FIG. 1, use a tuning mechanism, as shown in cross section in FIG. 5; in this configuration the capacitive coupling between the vertical probe (81) and the central conductor (83) of the slotted airline (slabline, 77, 78) creates a wideband reflection, that can be adjusted by changing the distance between the bottom of the probe (81) and the central conductor (83).
The modifications of both the amplitude and phase of the reflection factors due to the capacitance change are wideband in nature and generate reflection factors at several harmonic frequencies. When the tuner is moving, the impedances at all harmonic frequencies change simultaneously, and since all harmonic reflections are generated by the same wideband RF probe in the tuner, there is a natural and fixed relationship between them.
This makes it impossible to control the impedances at the harmonic frequencies independently. However, independent tuning of the impedances at the harmonic frequencies is a key requirement of a harmonic load pull test system. Therefore, it is impossible to perform harmonic load pull without the use of frequency path separation before the tuners by means of frequency discriminators, such as di- or tri-plexers, which would separate the signals at the various harmonic frequencies of interest, before they reach the tuners.
Active tuners (FIG. 2) are assemblies of microwave circuits and components that include at least one microwave amplifier, a phase shifter and a variable attenuator. These assemblies sample signals coming out of the DUT, using typically a directional coupler, and return them to the DUT after amplifying the signals and modifying their amplitude and phase. This generates a “virtual” reflection factor for the DUT. Active tuners use band-pass filters in their circuits and are, therefore, able to separate the various frequencies coming out of the DUT and thus synthesize harmonic impedances independently. Their disadvantage lies in much higher complexity and price than passive tuners, as well as power limitations and tendency for spurious oscillations.
As mentioned before, in order to manipulate the harmonic impedances, a passive load pull test system needs either some kind of frequency separation, between the tuners and the DUT or frequency selective tuners. In the absence of frequency selective tuners, frequency separation is achieved by using frequency discriminators, like frequency diplexers or triplexers. These components must be connected between the DUT and the actual tuner, introducing insertion loss that reduces the effective reflection factor effectively presented by the tuners to the DUT.
Frequency selective tuners, on the other hand, could be connected directly to the DUT and synthesize harmonic frequencies independently. At this point of time, frequency selective tuners, capable of changing the amplitude and phase of reflection factors at harmonic frequencies and capable of being used in harmonic load pull set-ups, are not known.